Split image, dual spectrum optical scanning system

ABSTRACT

A split image, dual spectrum optical scanning system includes a rotating multi-sided mirror for receiving radiant energy and for dividing the energy into split images. The split images are reflected through equal and opposite relay mirror systems to a Cassegrain type optical system, which converges the split images into a narrow envelope. An angularly extending dichroic mirror reflects the ultraviolet/visible portions of the split images out of the envelope to an ultraviolet/visible detector. The dichroic mirror also passes the infrared portions of the split images to an angularly extending mirror that reflects the infrared portions to an infrared detector.

United States Erickson 154] SPLIT IMAGE, DUAL SPECTRUM OPTICAL SCANNINGSYSTEM [72] Inventor: John G. Erickson, Forth Worth, Tex.

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

[22] Filed: June 3, 1970 [21] Appl. No.: 43,053

UNITED STATES PATENTS 3,211,046 10/1965 Kennedy...

151 Mar. 21, 1972 OTHER PU BLlCATlONS A Telescope Suitable forRocket-Borne Instrumentation; P. H. Verdone, Applied Optics, Vol. 6, No.3; March 1967; pg. 570 & 571.

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-Samuel M. Mims, Jr., James 0. Dixon, Andrew M. Hassell, HaroldLevine and Rene E. Grossman [5 7] ABSTRACT A split image, dual spectrumoptical scanning system includes a rotating multi-sided mirror forreceiving radiant energy and for dividing the energy into split images.The split images are reflected through equal and opposite relay mirrorsystems to a Cassegrain type optical system, which converges the splitimages into a narrow envelope. An angularly extending dichroic mirrorreflects the ultraviolet/visible portions of the split images out of theenvelope to an ultraviolet/visible detector. The dichroic mirror alsopasses the infrared portions of 3,495,029 2/1970 Underhlll the Splitimages to an angularly extending mirror that reflects the infraredportions to an infrared detector,

6 Claims, 1 Drawing Figure l I8 739 y 36501329 x OR IN ass/v4- PatentedMarch 21, 1972 3,650,629

V INVENTOR JOHN G. ERICKS ON @zmjw I I ATTORNH SPLIT IMAGE, DUALSPECTRUM OPTICAL SCANNING SYSTEM This invention relates to a splitimage, dual spectrum optical g scanning system, and more particularly toan optical scanning system for use in airborne infrared andultraviolet/visual imaging systems.

Airborne imaging systems are used in both military and industrialapplications to obtain topographical data relating to particulargeographic areas. In military applications, these systems are used toobtain data relating to areas that are otherwise inaccessible, eitherbecause they are controlled by hostile forces or because they arephysically inaccessible. In industrial applications, airborne imagingsystems are used in such diverse fields as agriculture, forestry,geography, geology, hydrology, and oceanography to obtain topographicaldata more economically and more rapidly than is possible by othermethods.

In most airborne imaging systems, an optical scanning system is used tocollect energy radiated upwardly from the earth and to focus the energyonto one or more detectors. Both the operating characteristics and theperformance of an imaging system of this type are directly related tothe design of its optical scanning system. For this reason, researchdirected toward the improvement of airborne imaging systems has resultedin the development of a variety of optical scanning systems. Theseinclude whole image systems, split image systems and split image, dualspectrum systems.

In a typical whole image optical scanning system, a wedgeshaped scanningmirror is rotated about an axis extending parallel to the surface of theearth. Energy radiating upwardly from the earth is reflected by thescanning mirror onto a collecting mirror. The collecting mirror in turnfocuses the energy onto a detector.

Whole image scanning systems of the wedge-shaped scanning mirror typeprovide a constant optical aperture. This is important, because aconstant optical aperture is essential to the production of a highquality output in an aircraft imaging system. However, whole imagescanning systems also exhibit certain inherent disadvantages. Forexample, whole image scanning systems of the wedge-shaped scanningmirror type are generally characterized by low scanning rates. When sucha system is employed in an airborne imaging system, this characteristicnecessitates the rotation of the wedge-shaped scanning mirror at a veryhigh speed. High speed rotation of the scanning mirror producescentrifugal forces that tend to distort the mirror, which in turndistorts the output of an airborne imaging system.

The inherent disadvantages of whole image optical scanning systems ledto the development of the split image optical scanning system disclosedand claimed in US. Pat. No. 3,21 1,046 granted to Howard V. Kennedy onOct. 12, 1965, and assigned to Texas Instruments Incorporated. Inaccordance with the Kennedy invention, a multisided scanning mirror isrotated about an axis extending approximately parallel to the surface ofthe earth. The multisided scanning mirror divides energy radiatingupwardly from the earth into two split images. The split images arereflected through equal and opposite relay mirror systems, and arerecombined in a detector.

The use of a split image optical scanning system in an airborne imagingsystem results in several advantages over the use of a whole imagesystem. For example, because of its smaller size, the multisidedscanning mirror of a split image system is not distorted by centrifugalforces. More importantly, split image optical scanning systems achieve aphenomenon known as optical doubling, and therefore operate at aconsiderably higher scanning rate than is possible with whole imagescanning systems. This split image optical scanning system disclosed inthe above-identified Kennedy Patent also maintains the constant opticalaperture that is characteristic of the prior wedge-shaped scanningmirror type whole image optical scanning system.

The above-identified Kennedy patent also discloses a split image, dualspectrum optical scanning system. In the latter system, the split imagesformed by the multisided scanning mirror are reflected onto a dichroicmirror. The dichroic mirror passes the portions of the split imageshaving wavelengths in the infrared spectrum to an infrared detector, andreflects the portions of the split images having wavelengths in theultraviolet/visible spectrum to an ultraviolet/visible detector. Theinfrared and ultraviolet/visible portions of the split images arerecombined in the infrared and ultraviolet/visible detectors,respectively, so that the split image, dual spectrum optical scanningsystem simultaneously produces both infrared and ultraviolet/visibleoutputs. Preferably, the detectors are so positioned that energyradiated from a particular point on the earth is simultaneously receivedby both detectors.

Unfortunately, the split image, dual spectrum optical J scanning systemsdeveloped heretofore have not provided the constant optical aperturethat is characteristic of whole image and split image scanning systems.In a typical prior split image, dual spectrum system, this deficiency iscaused by the positioning of the supporting structure for theultraviolet/visible detector in the path of portions of the split imagesas the split images are reflected through the system. This causes theenergy received by the detectors of the system to vary with the positionof the multisided scanning mirror. That is, because portions of thesplit images are blocked, the optical aperture of the system changesduring the rotation of the multisided mirror.

The present invention relates to a split image, dual spectrum opticalscam i ng system which provides a constant optical aperture, andmierefore superior to prior split image, dual spectrum scanning systemsfor use in airborne imaging systems. In accordance with the preferredembodiment of the invention, a Cassegrain type optical system isemployed to focus split images onto an angularly extending dichroicmirror, which reflects the ultraviolet/visible portions of the splitimages to an ultraviolet/visible detector positioned laterally of thescanning system. The dichroic mirror also passes the infrared portionsof the images to an angularly extending mirror, which reflects theinfrared portions of the images to an infrared detector also positionedlaterally of the scanning system. No portion of either of the splitimages is blocked as the split images are reflected through the scanningsystem, and the system therefore provides a constant optical aperture.

A more complete understanding of the invention may be had by referringto the following detailed description when taken in conjunction with thedrawing, wherein a split image, dual spectrum optical scanning systememploying the present invention is schematically illustrated.

Referring now to the drawing, a split image, dual spectrum Wemployingthe present invention is shown. The scanning system 10 includes arptating multisided scanning mirror 12 which receives energy rammearthafid divides the energy into split images. In the embodiment shownin the drawing, the mirror 12 is a foursided mirror. It will beunderstood, however, that the mirror 12 may comprise three, four, ormore sides in accordance with the requirements of a particularapplication.

The split image, dual spectrum optical scanning system 10 furtherincludes a pair of equal and opposite relay mirror systems 14 and 16,each including a first folding mirror 18 and a second folding mirror 20.The relay mirror systems 14 and 16 reflect the split images generated bythe scanning mirror 12 onto an ellipsoidal primary mirror 22 whichconverges the split images toward a focal point F. The converging splitimages are then reflected back toward the primary mirror 22 by aspheroidal secondary mirror 24.

The mirrors 22 and 24 comprise a Cassegrain type optical system whichconverges the split images formed by the scanning mirror 12 into anarrow envelope centered on the optical axes of the mirrors 22 and 24. Adichroic mirror 26 is positioned in the envelope, and therefore receivesthe split images from the secondary mirror 24. The dichroic mirror 26extends angularly relative to the envelope and reflects the portions ofthe split images in the ultraviolet/visible spectrum through a suitablerelay lens system 28 to an ultraviolet/visible detector 30 positionedlaterally of the scanning system 10. The detector 30 receives theultraviolet/visible portions of both of the split images, and producesan output indicative of the intensity of the ultraviolet/visible portionof the radiation received by the scanning mirror 12.

The dichroic mirror 26 also passes the portions of the split images inthe infrared spectrum to an angularly extending mirror 32. The mirror 32reflects the infrared portions of the split images through a suitablerelay lens system 34 to an infrared detector 36 also positionedlaterally of the scanning system 10. The infrared detector 36 receivesthe infrared portions of both of the split images, and produces anoutput indicative of the intensity of the infrared portions of all ofthe radiation received by the scanning mirror 12.

Assuming that the multisided scanning mirror 12 of the split image, dualspectrum optical scanning system rotates in a counterclockwisedirection, the split image received by a relay mirror system 16 from thescanning mirror 12 varies from minimum to maximum during rotation of themirror 12, and the split image received by the relay mirror system 14varies from maximum to minimum. However, this maximizingminimizingeffect is equal and opposite in the mirror systems 14 and 16 and iscancelling in nature, so that the total energy transmitted from themirror 12 to the detectors 30 and 36 is constant. As is clearlyindicated by the dashed lines in the drawings, the mirrors 26 and 32 arepositioned within the paths of the split images as the split images arereflected through the scanning system 10. That is, no portion of eitherof the split images is blocked as the split images are reflected throughthe scanning system 10 from the scanning mirror 12 to the detectors 30and 36. The scanning system 10 therefore provides a constant aperture,because the detectors 30 and 36 continuously receive all of theirrespective portions of the energy received from the scanning mirror 12.

The detectors 30 and 36 preferably comprise conventional radiationdetector systems. In accordance with particular requirements, thedetector 30 may be responsive to the entire ultraviolet/visiblespectrum, to the ultraviolet spectrum only, to the visible spectrumonly, or to a specific portion of either the ultraviolet spectrum or thevisible spectrum. Furthermore,

the outputs necessary in certain applications of the scanning system 10may require the use of a detector 30 responsive to the short wavelengthportions of the infrared spectrum. Similarly, the detector 36 may beresponsive to the entire infrared spectrum or to specific portionsthereof, as required.

The use of the present invention is advantageous over the prior art forseveral reasons. First, the system provides a constant optical aperture,which is virtually essential to the high quality performance of airborneimaging systems. Second, the system provides a dual spectrum output,which is invaluable in many of the military and industrial applicationsof airborne imaging systems. Third, the system permits the positioningof detectors laterally of the system proper. This facilitates both theuse of detectors of any convenient size and the interchanging of thedetectors to permit the detection of specific wavelengths within theinfrared spectrum and within the ultraviolet/visible spectrum.

Although a specific embodiment of the invention is illustrated in thedrawing and described herein, it will be understood that the inventionis not limited to the embodiment disclosed, but is capable ofrearrangement, modification and substitution of parts and elementswithout departing from the spirit of the invention.

What is claimed is:

l. A split image, dual spectrum optical scanning system comprising:

a multi-sided mirror and means for rotating the multi-sided mirror aboutan axis extending longitudinally through the mirror,

a concave mirror-convex mirror optical system for converging the splitimages into an envelope and equal and opaccording to claim 1 wherein thedichroic mirror and the reflecting means reflect their respectiveportions of the split images to detectors positioned laterally of thescanning system.

3. A split image, dual spectrum optical scanning system and for dividingthe radiated energy into split images, means for directing the splitimages along a path, said means including a Cassegrain type opticalsystem comprising a concave primary mirror and a convex secondary mirrorwherein the concave primary mirror converges the split images toward afocal point,

a dichroic mirror mounted in the path between the converging splitimages and extending angularly with respect thereto for reflectingpredetermined portions of the split images out of the path to a detectorpositioned laterally of the scanning system,

a mirror mounted in the path and extending angularly with respectthereto for reflecting the other portions of the split images out of thepath to a second detector positioned laterally of the scanning system.

4. The split image, dual spectrum optical scanning system according toclaim 3 further including relay lens systems for focusing thepredetermined portions and the other portions of the split images on thedetector and the second detector respectively.

5. In a split image optical scanning system of the type including arotating multi-sided mirror for dividing received radiant energy intosplit images, equal and opposite relay mirror systems for receiving thesplit images from the multi-sided mirror and a first curved concavemirror for receiving the split images from the relay mirror systems andfor converging the split images toward a focal point, the improvementcomprising:

a second curved convex mirror for receiving the split images from thefirst curved concave mirror and for converging the split images into anarrow envelope, wherein the first curved mirror and the second curvedmirror comprises a Cassegrain type optical system,

a dichroic mirror, extending angularly with respect to the envelope andpositioned between the converging paths of the split images, forreceiving the split images from the second curved mirror and forreflecting predetermined portions of the split images out of theenvelope, and

a mirror extending angularly with respect to the envelope for receivingother portions of the split images from the second curved mirror and forreflecting the other portions of the split images.

6. The split image optical scanning system according to claim 5 furtherincluding a pair of detectors, a relay lens system for receiving thepredetermined portions of the split images from the dichroic mirror andfor focusing the predetermined portions of the split images on one ofthe detectors, and a relay lens system for receiving the other portionsof the split images and for focusing the other portions of the splitimages on the other of the detectors.

c a rotating multhsided ntirror for receiving radiated energy 1 l

1. A split image, dual spectrum optical scanning system comprising: amulti-sided mirror and means for rotating the multi-sided mirror aboutan axis extending longitudinally through the mirror, a concavemirror-convex mirror optical system for converging the split images intoan envelope and equal and opposite mirror relay systems for reflectingthe split images from the rotating multi-sided mirror to the concavemirror-convex mirror optical system, and a dichroic mirror mounted inthe envelope between the converging split images for passingpredetermined portions of the split images and for reflecting otherportions of the split images out of the envelope and means mountedbeyond the dichroic mirror for reflecting the predetermined portions ofthe split images out of the envelope.
 2. The split image, dual spectrumoptical scanning system according to claim 1 wherein the dichroic mirrorand the reflecting means reflect their respective portions of the splitimages to detectors positioned laterally of the scanning system.
 3. Asplit image, dual spectrum optical scanning system comprising: arotating multi-sided mirror for receiving radiated energy and fordividing the radiated energy into split images, means for directing thesplit images along a path, said means including a Cassegrain typeoptical system comprising a concave primary mirror and a convexsecondary mirror wherein the concave primary mirror converges the splitimages toward a focal point, a dichroic mirror mounted in the pathbetween the converging split images and extending angularly with respectthereto for reflecting predetermined portions of the split images out ofthe path to a detector positioned laterally of the scanning system, amirror mounted in the path and extending angularly with respect theretofor reflecting the other portions of the split images out of the path toa second detector positioned laterally of the scanning system.
 4. Thesplit image, dual spectrum optical scanning system according to claim 3further including relay lens systems for focusing the predeterminedportions and the other portions of the split images on the detector andthe second detector respectively.
 5. In a split image optical scanningsystem of the type including a rotating multi-sidEd mirror for dividingreceived radiant energy into split images, equal and opposite relaymirror systems for receiving the split images from the multi-sidedmirror and a first curved concave mirror for receiving the split imagesfrom the relay mirror systems and for converging the split images towarda focal point, the improvement comprising: a second curved convex mirrorfor receiving the split images from the first curved concave mirror andfor converging the split images into a narrow envelope, wherein thefirst curved mirror and the second curved mirror comprises a Cassegraintype optical system, a dichroic mirror, extending angularly with respectto the envelope and positioned between the converging paths of the splitimages, for receiving the split images from the second curved mirror andfor reflecting predetermined portions of the split images out of theenvelope, and a mirror extending angularly with respect to the envelopefor receiving other portions of the split images from the second curvedmirror and for reflecting the other portions of the split images.
 6. Thesplit image optical scanning system according to claim 5 furtherincluding a pair of detectors, a relay lens system for receiving thepredetermined portions of the split images from the dichroic mirror andfor focusing the predetermined portions of the split images on one ofthe detectors, and a relay lens system for receiving the other portionsof the split images and for focusing the other portions of the splitimages on the other of the detectors.